I was over on the South Sound this week, and found this insect shuck floating on the water. We were within perhaps 100 yards of a good sized stream emptying into the bay, on grassy flats extending out into the salt, with acres of mudflats all around during low tide (we were there on a high). It sure looks a lot like a giant stonefly, but that's not something I would expect to find where we were. For scale, that Buff logo is about 1" on the long axis.

I agree that it is a stonefly molt. As stoneflies crawl out onto the shore to molt, their air-filled molts could be carried off when rising water (and we've had that recently....) inundated the stream bank. A dry, air-filled molt could easily be carried from the banks of one of our streams into the estuary where you found it. And you are correct that there are no marine stoneflies.
As a side-note, because stoneflies are very sensitive to water quality, the fact that you found this molt would indicates that the adjacent stream (if that was the source) flows with unpolluted water.
Steve

I take it then that we're in agreement? If @Taxon thinks it's a stonefly, it's a stonefly. I'd expect them to be hatching later in the year, would this be a molt?

Big sucker, eh?

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Hi P,
It looks to me like the terminal molt from the aquatic stage to a flying adult. There are a number of stoneflies (for example, Skwalas and winter stones) that emerge in the winter and early spring. The giant stoneflies tend to be later and golden stoneflies in early summer and little yellow (yellow Sallie) later in summer.
Steve

I was fishing the Foss river once and as I was looking around I saw these same casings stuck on the bridge support. I've never seen a bug as big as the casings were. There were two of them side by side. This was over 20 years ago.

FlyFishingEntomology.com

Helping to fill a gap here on WFF with Entomology will be Roger Rohrbeck (Taxon). Roger has an impressive piece of work with FlyfishingEntomology.com and has consistently been the source of answers on the subject here on WFF. Rather than overlap efforts on the subject we'll have this forum area dedicated to subject with Roger moderating. Note the links below to his site. Thanks.